As is pointed out in the co-owned U.S. Pat. No. 4,399,028, in which one of the present applicants was a co-inventor, froth flotation apparatus is well-known for the separation of small particles from liquid suspension, and the assignee of the present invention is particularly interested in the development and manufacture of froth flotation apparatus for use in the de-inking of waste paper furnishes so that the resulting stock can be reused for the manufacture of paper. For example, the paper making industry presently has great interest in the de-inking of waste white papers, such as ledger papers, to produce a stock which can be used in the manufacture of tissue, and also in the de-inking of used newsprint.
The above patent discloses froth flotation apparatus comprising a generally cylindrical side wall and internal partitions which cooperate to define a plurality of vertically stacked, annular flotation cells surrounding a common discharge conduit for foam. The stock suspension to be de-inked or otherwise cleaned is supplied to the top cell by an inlet which includes provision for aerating the stock as it is delivered into the cell. The contaminant-containing foam which develops on the top of the liquid in the uppermost cell is skimmed off into the foam discharge conduit, while the stock cleaned in passing through that cell is delivered to the next lower cell, again accompanied by aeration, and the same process is repeated in each of the lower cells successively as the stock passes therethrough to the outlet conduit from the lowermost cell.
In that patent, the mechanism for aerating the stock as it is delivered into each cell, which is identified as a "sparger", comprises a hollow cylinder member of porous material, such as sintered bronze, mounted for rotation in the path of the stock suspension being delivered to each cell on an axis extending transversely of the inlet conduit to the cell. Compressed air is supplied to the interior of this cylinder while it is being rotated, and passage of the air through and discharge from the pores in the cylinder produce bubbles which effect the desired aeration of the stock and the creation of foam within the cell.